The cloning of at least fifteen subtypes of P2 nucleotide receptors has presented a unique challenge to medicinal chemists: the design of selective agonists and antagonists for this multiplicity of receptors with few existing leads. These receptors regulate function of the central nervous system, the immune system, the cardiovascular system, and smooth muscles. Our laboratory is developing selective agonists and antagonists for these receptors, for use both as pharmacological tools for probing receptor function and as potential therapeutic agents. P2X receptors are ligand-gated ion channels. P2Y receptors are G protein coupled receptors linked to the phosphatidyl inositol pathway as second messenger. The human P2Y1 receptor as representative of the P2Y family of metabotropic purine and pyrimidine nucleotide receptors may be modeled based on a rhodopsin template, and the resulting model is highly consistent with pharmacological and mutagenesis results. Charged residues in both the transmembrane and extracellular domains and two disulfide bridges essential for receptor activation have been identified. Selective P2Y1 receptor antagonists such as the adenine nucleotide MRS 2179 (N-methyl-2'-deoxyadenosine-3',5'-bisphosphate) and it carbocyclic analogue are under development. We have also synthesized nucleotides containing conformationally constrained ribose-like rings, in order to freeze a conformation that provides favorable affinity and/or selectivity at P2 receptors. As a result, we have identified the conformation preference of the P2Y1 receptor for the Northern ring conformation of the ribose. This conclusion applies to both agonists and antagonists. By freezing the ribose substitute in the receptor-preferred conformation, we have enhanced the potency of known agonists at the P2Y1 subtype by 200-300 fold. One ATP derivative containing a methylene carbon joining the second and third phosphate groups was qualitatively altered in its effects on the P2Y1 receptor: The ribose analogue is inactive, and the confromationally constrained analogue (Northern methanocarba) was a moderately potent agonist. MRS 2500, a 2-iodoadenine-methanocarba bisphosphate nucelotide, in which the ribose-like ring is locked in the North conformation is the most potent known antaognist of the P2Y1 receptor. This antagonist has been shown to potently inhibit the ADP-induced aggregation of human platelets. MRS 2365, a (N)methanocarba analogue of 2-methylthio-ADP, is the first selective P2Y1 agonist and was shonwn to induce a shape change in platelets without producing aggregation. This supports the view that both P2Y1 and P2Y12 receptor activation are needed for ADP-induced aggregation. These pharmacological tools will be useful in validating the possible use of P2Y1 antagonists as antiplatelet agents. The use of conformationally constrained nucleotides has also been extended to P2Y2, P2Y4, P2Y6, and P2Y11 subtypes. The Northern methanocarba ring system results in retention of high potency in agonists at all of the above subtypes, except P2Y6. The conformational requirements of the P2Y6 receptor are currently being explored in our section. Also, a relationship between this subtype and apoptosis, programmed cell death, has been discovered. Astrocytoma cells that express the P2Y6 receptor, when activated by UDP, are protected from apoptosis induced in control cells upon exposure to TNF, tumor necrosis factor. The protection involves activation of protein kinase C and subsequently the signaling kinase known as ERK. This may have relevance for degenerative and inflammatory conditions that involve TNF. A highly potent and selective antagonist of the P2Y1 subtype has been developed using a similar conformational strategy (MRS 2279, which also contains a Northern methanocarba ring system in place of ribose). This compound has been shown to inhibit the ADP-induced aggregation of both rat and human platelets. Its tritiated version is the first high affinity radioligand for the P2Y1 receptor, of general applicability, to be reported. We are continuing the explore the structure activity relationships in this series of potent and selective P2Y1 receptor antagonists. Modeling of P2X receptors has not been achieved, since no template for the extracellular nucleotide binding region exists. Nevertheless, a selective antagonist, MRS 2220, and a potentiator, MRS 2219, of this subtype have been identified. Both are based structurally on pyridoxal-5'-phosphate antagonists (such as PPADS), for which the SAR is being examined at all of the P2 receptor subtypes.